1. Field of the Invention
The present invention is related to a Gaussian frequency shift keying/frequency shift keying (GFSK/FSK) modulating circuit and related method, and more particularly, to a modulation circuit and related method for implementing a GFSK/FSK in a digital manner.
2. Description of the Prior Art
Since electromagnetic waves were discovered in the end of the 19th century, the development of wireless communications has been changing with each passing day. Wireless communications has almost become a necessity whether in the field of business application or in everyday life. The frequency range of human hearing is about from 20 Hz to 20 kHz and it is difficult to transmit information via such low-frequency radio waves. Therefore, information transmission can be achieved by emitting higher-frequency radio waves (called Carriers) and concealing desired information into the carriers from which receivers can recover the messages. Through modulating and demodulating signals, information can be transmitted to great distances.
Bluetooth is a low-power and short-range wireless transmission technology, operating in a frequency band of 2.4 GHz. Many devices are inexpensive and can directly work without additional modifications on the original basic design. Bluetooth makes use of a Frequency Hopping Spread Spectrum (known as FHSS) technique to transmit and receive data and a Gaussian Frequency Shift Keying (known as GFSK) technique to modulate signals. Because GFSK modulation not only can efficiently reduce the transmission bandwidth but also is a low-cost modulation technique able to be implemented by simple IC design, it is widely adopted in wireless communications systems.
Please refer to FIG. 1. FIG. 1 illustrates a schematic diagram of a prior art GFSK modulation circuit 10. The GFSK modulation circuit 10 includes a pulse-shaping filter 12, a voltage controlled oscillator VCO, a loop filter, a reference oscillator XTAL, a phase/frequency detector 17, and a charge pump 18. A baseband modulating signal Sm first passes through the pulse-shaping filter 12, and then inputs to the voltage controlled oscillator VCO. The voltage controlled oscillator VCO generates a feedback signal So that inputs to the phase/frequency detector 17. The reference oscillator XTAL generates a reference clock signal Si that inputs to the phase/frequency detector 17. The phase/frequency detector 17 has a first input end 172 coupled to an output of the reference oscillator XTAL for receiving the reference signal Si and a second input end 174 coupled to an output of the voltage controlled oscillator VCO for receiving the feedback signal So. The phase/frequency detector 17 functions to estimate the phase difference between the input signals Si and So from the two corresponding input ends 172 and 174 so as to generate an error signal Se. When the reference signal Si has a phase lead to the feedback signal So, the error signal Se is an Up signal. When the reference signal Si has a phase lag to the feedback signal So, the error signal Se is a Down signal.
Please keep referring to FIG. 1. The input end 182 of the charge pump 18 coupled to the output end of the phase/frequency detector 17 functions to output a control current to the loop filter 14 in accordance with the received error signal Se. When the error signal Se is an Up signal, the charge pump 18 supplies more electricity to the loop filter 14. When the error signal Se is a Down signal, the charge pump 18 supplies less electricity to the loop filter 14. The loop filter 14 is a simple circuit design commonly comprising a capacitor which charges or discharges in accordance with the error signal Se. The loop filter 14 low-pass-filters the output control current of the charge pump 18 and generates a corresponding control voltage Sc. The voltage controlled oscillator VCO includes a first input end 124 coupled to an output end of the pulse shaped filter 12 and a second input end 144 coupled to an output end of the loop filter 14 for receiving the control voltage Sc in accordance with which the VCO adjusts the input signal of the first input end 124 to generate the feedback signal So.
As illustrated in FIG. 1, the feedback signal So performs at a specific frequency controlled by the control voltage Sc and furthermore feedbacks to the phase/frequency detector 17, which constantly estimates the phase difference between the received reference clock signal Si and the received feedback signal So. The loop filter 14 continues adjusting the control voltage Sc after receiving the control current outputted by the charge pump 18. Therefore, the voltage controlled oscillator VCO can continuously adjust the frequency of the feedback signal So in order to reduce the phase difference between the reference clock signal Si and the feedback signal So. Integrating the voltage controlled oscillator VCO, the loop filter 14, the phase/frequency detector 17, and the charge pump 18 constructs a feedback electronic circuit called a Phase Lock Loop, PLL. Thus the voltage controlled oscillator VCO driven by the control voltage Sc adjusts its own output frequency to ideally reduce the phase difference. By duplicating and tracking the phase and frequency of its own input again and again, the VCO enables the Phase Lock Loop to lock at the ideal desired frequency.
As illustrated in FIG. 1, the GFSK modulation circuit 10 further comprises a switch SW1 coupled between the charge pump 18 and the loop filter 14. When the Phase Lock Loop begins to process modulation, the switch SW1 switches on. When the Phase Lock Loop begins to do phase locking, the switch SW1 switches off, thereby capable of avoiding the frequency shift when the frequency of the baseband modulating signal Sm is too low.
Among prior arts, it is common to implement modulation circuits in an analog manner. In general, the baseband modulating signal Sm directly drives the voltage controlled oscillator VCO, but in order to achieve GFSK modulation, the baseband modulating signal Sm beforehand passes through a pulse shaped filter 12 that commonly comprises resistances, inductances, and capacitors. The shortcoming of the pulse shaped filter 12 is that the modulation parameters such as a modulation index and a pulse shaped factor cannot precisely be controlled. Besides, in order to avoid the frequency shift when the frequency of the baseband modulating signal Sm goes too low, the open-loop control, SW1 as shown in FIG. 1, is needed, resulting in the higher complexity of the circuit design and the longer time consumption.